Tap changers for uninterrupted changeover between different winding taps of tapped transformers have been in use in large numbers worldwide for many years. Such tap changers usually consist of a selector for power-free selection of the respective winding tap that is to be switched to, of the tapped transformer and a load-changeover switch for the actual switching from the connected to the new, preselected winding tap. The abrupt changeover usually takes place with the assistance of an force-storage device whose triggering rapidly rotates a switch shaft. In addition, the load-changeover switch usually comprises a contact switch s and a resistance switch. The contact switches in that case serve for direct connection of the respective winding tap with the load diverter and the resistance switches for temporary connection, i.e. bridging-over by one or more switch-over resistances.
In such a load-changeover switch of a tap changer, the vacuum-switching tubes are used for uninterrupted switching, is known from DE 195 10 809 [U.S. Pat. No. 5,834,717]. In that case respective cam disks are provided for each switching element to be actuated and each movement direction of the drive shaft. The respective ends of the individual cam disks have a defined profile that departs from the circular shape and that, on rotation of the switch shaft, actuate individual vacuum-switching tubes or also mechanical contacts.
In DE 42 31 353 A1, actuation of the individual switching tubes is effected by a switch shaft that is rotatable in both directions and that is rapidly rotated after triggering of a force-storage device. In that case, fixed on the switch shaft for actuation of the vacuum-switching tubes is a cam disk on whose end there is for each vacuum-switching tube a control cam in which a roller—that acts on the actuating lever of the associated vacuum-switching tube—is mechanically positively and constrainedly guided. The control cam is here realized in the form of a horizontal encircling groove that departs from a circular profile and in which the respective roller mechanically positively engages.
In addition, there is known from DE 40 11 019 [U.S. Pat. No. 5,107,200] a load-changeover switch that operates according to the reactor principle and in which the cam disk for actuation of the contacts does not have at the encircling end a profile departing from a circular shape, but possesses grooves of geometrically different design on the upper side and lower side thereof. A double-sided actuation of different switching elements with different switching sequence is possible with this known solution.
However, in different cases of use of such known tap changers with vacuum-switching tubes for regulation of power transformers a high surge voltage strength of up to 100 kV and significantly thereabove is required. Such undesired surge voltages whose level is essentially dependent on the construction of the tapped transformer and the winding parts between the individual winding taps, are on the one hand lightning surge voltages resulting from lightning strikes in the grid. On the other hand, switching surge voltages caused by unpredictable switching surges in the grid to be regulated can also occur.
If the tap changer has insufficient surge voltage strength a temporary tap short-circuit or undesired break-through of the ceramic or of the damping screens of vacuum-switching tubes in the load branch not conducting load current can happen that not only can cause long-term damage thereof, but also is undesirable in general.
This problem is solved in the prior art inter alia in that the movable plunger of the vacuum-switching tube, which is susceptible in terms of circuitry to this lightning surge voltage, is provided with a stroke of such a length, i.e. spacing between the contact surfaces of the co-operating fixed and movable plungers within the vacuum-switching tubes, that the dielectric spacings between the contact surfaces of the co-operating plungers are thereby dimensioned to be sufficiently large and as a consequence undesired arcing during a lightning strike surge cannot arise.
However, a disadvantage of the solution known from DE 42 31 353 A1 is that the required long stroke of the movable plunger requires a control cam with correspondingly large cam throws. This in turn presupposes within the load-changeover switch a large constructional space that, however, is often not available and from the technical aspect is a constructionally disadvantageous solution. Moreover, in each switchover process there is material erosion at contact surfaces of the contact system within the vacuum-switching tube caused by electric arcs that occur during mechanical separation of the contact surfaces of the vacuum-switching tube under load. DE 42 31 353 A1 does not offer a solution as to how compensation can be provided for the effects that are produced by material erosion of the contact surfaces on the actuating lever of the respective vacuum-switching tube and that with time produce tolerances within the contact system. This problem comes very much to the forefront due to the promised lengthy maintenance intervals of several hundred thousand switch-over actions per tap changer that manufacturers guarantee to customers in the case of a tap changer with vacuum-switching tubes.
DE 195 10 809 C1 does, in fact, compensate for tolerances arising due to material erosion at the contact surfaces in that here the actuating lever operatively connected with the movable plunger of the vacuum-switching tube applies the requisite contact force purely by a spring and not by mechanically constrained guidance. In that case the spring has, apart from its function of providing the required contact force, the further task of introducing the closing force to the actuating lever. In other words: the spring here has a double function of compensation for tolerances and application of the required contact force in the closed state of the vacuum-switching tubes. If in this solution known from DE 195 10 809 C1 the stroke of the movable plunger of the vacuum-switching tube is to be increased then a larger size compression spring with a spring rate corresponding therewith and greater spring travel is necessary for that purpose. However, in sum this causes a significantly increased expenditure of force for actuation of the vacuum-switching tubes and thus a more substantial dimensioning of the individual components that in turn is linked with significant disadvantages due to constructional and cost aspects. Since, as just described, the contact force of the contact surfaces of the vacuum-switching tube is limited to the spring force of the spring employed, in the case of a short-circuit load a transient lifting-off of the contact surfaces of vacuum-switching tube and thus an electrically undefined state within the power transformer can additionally arise.